In automation and process control technology, field devices are used in many ways that measure process variables in the process sequence (sensors) or that control controlled variables (actuators).
Field devices for determining flow, fill level, differential pressure, temperature, etc. are widely known. For detecting the corresponding process variables, that is, mass or volume flow, fill height, pressure, temperature, etc., the field devices are disposed in the immediate vicinity of the process component involved.
The field devices furnish a measurement signal which corresponds to the measured value of the process variable detected. This measurement signal is carried to a central control unit (such as a control station or process control system). As a rule, the process control is done entirely from the control unit, where the measurement signals of various field devices are evaluated and as a function of the evaluation control signals for actuators that control the process sequence are generated.
As an example of an actuator, a controllable valve that regulates the flow of a liquid or gas in a pipeline segment can be named.
The signal transmission between the field device and the control unit can be done in analog or digital form (for instance via a current loop or a digital data bus).
Known international standards for signal transmission include for instance 4- to 20-mA current loops, HART, PROFIBUS, FOUNDATION FIELDBUS, or CAN bus.
Along with analog field devices, programmable field devices are being increasingly used. Often, ASICs (application-specific integrated circuits) and SMDs (surface mounted devices) are used for this purpose.
In the programmable field devices, more and more “intelligence” is thus shifted in the field to the actual site of use.
The applicable control program of the field device is stored in a nonvolatile memory in the field device and is executed in a microprocessor. This control program controls the operation, measurement and control functions, among others, of the field device.
In sensors that are used to detect measured values, normally larger control programs are used than in actuators that are used for only control purposes.
Because of the larger control programs, the versatility of the control programs in sensors is greater.
Various functionalities are possible; for instance, the way in which data is stored can vary; the calibration can be done with variable precision; data detection and data evaluation can differ; various regulating and adjusting functions can be employed; the type of signal transmission can differ; a more or less complicated self-monitoring (predictive maintenance) is possible, etc.
The field device, particularly as a sensor, no longer forwards merely a simple measurement signal but rather a prepared measurement signal, optionally with further additional information. The total of all these capabilities makes up the functionality of the field device.
For each functionality, a suitable control program (software) is required. The control program is normally stored in a nonvolatile memory in the field device.
The functionality of a field device thus depends decisively on the control program, which as a rule is made available by the manufacturer of the field device.
The control program can include certain basic functions or expanded functionalities.
The control program is normally implemented upon manufacture of the field device; that is, after production the functionality of the field device is fixedly specified and can no longer be altered in a simple way. In the event of program updates, or new functionalities (upgrades), replacement of the program is necessary.
One possibility is to replace the memory in which the control program is stored. This replacement must be done by skilled workers and is complicated.
Another possibility is not to replace the memory but instead to reprogram it. This can be done for instance with the aid of a portable operator control unit (such as a handbeld computer or PC), which is connected to the field device. Via a suitable interface in the field device, the new control program is transferred from the operator control unit to the field device and is stored in memory in the field device.
For some time, electrically erasable and reprogrammable memories, known as flash memories or flash EPROMS, that allow simple reprogramming have been known.
However, one disadvantage of such memories is that the reprogramming requires a power supply that supplies strong current, which is above the normal basic current for field devices, and so these special memories are not used in field devices.